Having some sort of prompt helps, too, to keep up the habit. I use Day One, and they have a prompt that changes daily; today's is "What is something I want to do for myself in the coming year?" One of my favorites is "Describe something you learned from your mother" which is absolutely not something I would have thought to write about unprompted.

Another cool thing they have is templates. I use one called "Daily Self", which just asks a bunch of questions about my day: Who did I see? What's my mood like? What are my accomplishments? What did I eat? (I added a couple more questions, what I read and what music I listened to).

Both of these have been effective tools for me. If I have something on my mind, I write about it and don't worry about prompts. If the daily prompt interests me, I'll answer it. And if there's nothing else, I just open the Daily Self template and write about my day.